Run your campaign like a TV serial.
* Keep your NPC's/secondary characters simple, building on 1 or 2 stereotypes or "tropes" as they're called in the TV industry.
* If you insist on running an adventure path, say, make sure that each "episode" ends in a proper spot. If you're just playing for sake of playing, try to keep the plots self-contained. By this, I mean each adventure should have one primary mission, one or two difficulties, one climax to the adventure, and one wrap-up scene.
* Not every adventure has to include killing things and taking their stuff. Depending on you and your player's preferred styles of play, it's very possible to spend an entire gaming session without rolling too many dice or causing too much bloodshed. One campaign I ran was very much like a sitcom where the "hero" was trying to bed all sorts of women and convince every wandering NPC to join his troupe (he was a newly minted King of a ruined kingdom - the land itself was fragmented and its population scattered). The cast of characters was memorable and they all fleshed themselves out as the hero developed his own persona.
* Don't limit your imagination, but be VERY CLEAR on what kind of game your solo player wants. It's much, much easier on you to design a world around him than have him fit into your Grand Design. I know that sounds backwards, but it's true. Also, since there's only one player, be prepared to play NPC's as if they were PC's in the way of advice, information, and combat.
* Don't be afraid to steal material from anywhere: books, tv, manga/anime, movies, newspapers, reality, other game systems... this is your chance to really explore your creativity, and no one expects you to crap out the Simarillion just to play some D&D.
I hope this helps. I've been playing and DM'ing solo campaigns (including playing through the entire Return to the Temple of Elemental Evil uber-module) for nearly a decade, and I use any and all of what I've said above to great effect.